Homogeneous-charge, compression-ignition combustion is known to those skilled in the art to provide high fuel efficiency and low emission operation in internal combustion engines. However, HCCI operation is feasible in a narrow range in engine torque, approximately one-third of the torque range of a conventional spark-ignited engine. Thus, most HCCI engines being developed are dual mode engines in which HCCI is used at low torque conditions. When a higher torque is desired, operation is transitioned to an alternative combustion mode, such as spark-ignition combustion or heterogeneous, compression-ignition combustion (diesel). Challenges accompanying such transitions include: torque matching (providing driver demanded torque during the transition interval), maintaining emission control, and robustly returning to HCCI combustion, to name a few. Another difficulty encountered in engines which transition from one combustion mode to another is that the combustion system geometry cannot be optimized for either combustion mode, but is instead a compromise. For example, a desirable compression ratio for HCCI combustion is about 15:1 and about 10.5:1 for spark-ignition combustion.
A disadvantage of HCCI combustion is its inferior transient response to a demand for a change in torque, orders of magnitude slower than SI combustion. The inventors of the present invention have recognized that HCCI operation cannot provide a vehicle operator with the responsiveness that they have come to expect from a SI engine.
In U.S. patent application 2004/0182359, an 8-cylinder HCCI/SI engine is described in which HCCI to SI transitions are made one cylinder at a time, i.e., at a lower torque demand all 8 cylinders are operating in HCCI combustion mode and as torque demand exceeds what HCCI combustion can provide, cylinders are individually switched to SI operation. The inventors of the present invention have recognized that it would be desirable to have an engine which provides the desired range in output torque at the high efficiency of HCCI combustion without having to undergo a combustion mode transition in any given cylinder because of the compromises inherent in designing a cylinder to operate robustly and efficiently in both HCCI and SI combustion modes over a wide operating range.